A typical zoom lens system consists of multiple lenses comprising a combination of movable and fixed lens groups. To change the magnification of the system, the movable lens groups move along the optical axis of the system. Each movable lens group moves to a pre-determined position along the optical axis so as to project a magnified image of an object to an image plane in an image pickup element such as a CMOS image sensor or a CCD camera. Thus, the movable lens groups always move in directions parallel to the optical axis.
As the magnification of the lens system changes during zooming, the field of view (FOV) of the zoom lens system changes. When an image is being magnified or it is viewed at higher resolution, the projection angles of the cone of rays from the object received onto the image pickup element are reduced and hence the FOV of the lens system is also reduced. Conversely, the FOV is larger for an image of lower resolution. At higher magnification, rays from the object projecting onto the image pickup element that are closer to the optical axis of the zoom lens system form a focused image since the rays going into the optical system are limited by the angular FOV of the zoom lens system which is in turn controlled by a field stop aperture of the optical system. A field stop aperture is commonly placed along the optical axis of the zoom lens system with its center at the optical axis.
Examples of zoom lens systems are disclosed in the prior art. U.S. Pat. No. 5,414,562 entitled “Zoom Lens” discloses a zoom lens system which achieves high efficiency in addition to a high magnification ratio for use in a compact zoom lens system. The compact zoom lens system comprises five lens groups wherein the second and fourth lens groups are movable along the optical axis so as to vary the magnification of the image. The other lens groups are at fixed positions.
In U.S. Pat. No. 7,006,300 entitled “Three-Group Zoom Lens”, first and second lens groups are movable along the optical axis for zooming whereas third and fourth lens groups do not move. Although this zoom lens system may be formed of only three lens groups, it satisfies specified conditions to ensure that the system is compact and favourably corrects various aberrations.
U.S. Pat. No. 6,292,306 entitled “Telecentric Zoom Lens System for Video Based Inspection System” discloses a zoom lens system incorporating telecentricity and consistent precise image resolution to maximize measurement accuracy. This is accomplished through the use of an adjustable aperture stop (telecentric stop) located at a focal point of the optical system and two movable groups of lenses for performing magnification zooming. The zoom lens groups are adjustable along the axial axis of the housing to alter the magnification of the image projected to a camera. A beamsplitter is mounted in the barrel and registers through an opening in the barrel with an external light source to direct light onto the surface of an inspected object, and to pass an image thereof to the zoom lens housing.
Thus, in the aforesaid conventional zoom lens systems, the construction of the optical systems rely on having different sets of movable lens groups moving along the optical axis of the optical system in order to capture images from light rays (paraxial rays) of an object on or in close proximity to the main optical axis. Accordingly, such traditional zoom lens systems can only increase or decrease magnification of images for which the light rays from the object are coming from the region immediately surrounding the optical axis of the lens system. With such conventional optical constructions, one is not able to obtain highly magnified images of the object from light rays remote from the main optical axis. Another limitation of the above conventional zoom lens systems is that the FOV available at higher magnification is narrower than at lower magnification. As a result, the area available for selecting a portion of an object of interest at high magnification is small.
Therefore, it would be desirable to devise an optical construction of a zoom lens system which is capable of magnifying an object and capturing the light rays from the object that are remote from the main optical axis of the lens system.